By directional solidification of silicon for production of multicrystalline silicon ingots a so-called “red zone” is formed along the outer part of the ingot, in the lower end of the ingot and at the top end of the ingot. The red zone is typically 2-3 cm thick and may be characterized by a short life time for minority carriers. The life time of minority carriers in the red zone area is measured to below the quality requirement of above 2 μs. The red zone area of directionally solidified ingots is therefore normally cut away and thus not used for wafers for solar cell production. The red zone area of the directional solidified multicrystalline silicon ingot reduces the yield of the ingot. The red zone appears both in polysilicon and in compensated silicon (silicon containing both boron and phosphorus) even though the red zone normally is thicker in compensated silicon than in polysilicon.
The reason of the formation of red zone at the lower end, along the walls and at the upper end of the directional solidified silicon ingots has been related to different kinds of defects; see Y. Boulfrad: Investigation of the Red Zone of multicrystalline Silicon Ingots for Solar cells; Doctoral Thesis at NTNU, Norway 2012:84. The main type of defect is caused by Fe and 0 diffusing into the solid silicon from the crucible and/or from the coating used in the crucible. Further defects such as dislocations and a synergistic effect between the different types of defects may also be present. Further there is a tendency that particles are present near the upper end of directional solidified silicon ingots. The upper part of the ingots must also for this reason be cut off and thereby limiting the yield.
In order to increase the yield of the silicon ingots it is desirable to minimize or totally avoid formation of the red zone, particularly in the lower end of the silicon ingots which would increase the useful part of the silicon ingots suitable for wafers and solar cell processing. It would further be advantageous if the content of particles near the upper end of the directional solidified silicon ingot could be reduced.